Django Read Replicas

This project aims to mimic the delay of replication between master and replica databases in cloud infrastructure (or elsewhere).

The repo has a few parts:

• docker-compose.yaml: Creates the Postgres Master and Read Replica
• Replica replicas/ directory: A directory that holds the logic required to allow Django to utilize the replica in tests
• Django Project: Demo of a project configured to use the replica mentioned above
• Django application: An application with tests that test for an expected level of delay

To get things running, follow these steps:

1. $ docker-compose up -d
2. $ docker exec -it django-read-replicas_master_1 /delay.sh
3. $ python manage.py test

If you need to specify varying levels of delay, pass a number of milliseconds of delay to the delay.sh script in the master container. 3000 is the default.

docker exec -it django-read-replicas_master_1 /delay.sh 5000

Troubleshooting

If you need to test that replication lag is in fact present, try the time-test.sh script in the root of this repo.

TODO

1. Explain project settings:

DATABASES[DB_ALIAS_MASTER]['AUTOCOMMIT'] = True

DB_ALIAS_REPLICA_1 = 'replica1'
DATABASES[DB_ALIAS_REPLICA_1] = {
    'ENGINE': 'django.db.backends.postgresql_psycopg2',
    'NAME': os.environ.get('PGDATABASE') or 'drr',
    'USER': os.environ.get('PGUSER') or 'drr_user',
    'PASSWORD': os.environ.get('PGPASSWORD') or 'pass',
    'HOST': os.environ.get('PGHOST_REPLICA_1') or 'localhost',
    'PORT': int(os.environ.get('PGPORT_REPLICA_1') or 5433),
    'TEST': {
        # Non-standard Django DATABASES > TEST key for use in DRR
        'REPLICA': True,
    }
}

DATABASE_ROUTERS = ['replicas.db.MasterReplicaRouter']

TEST_RUNNER = 'replicas.test.DiscoverRunnerWithReadReplicas'

2. Explain how to setup a test like the one in app/tests.py. Ie. Explain what the db_wait method does and why.
3. Package project for installation from PyPI.
4. Write article about this tool and why it was created